Microwave monolithic integrated circuit (MMIC) carrier interface

ABSTRACT

A microwave monolithic integrated circuit (MMIC) assembly and related method are disclosed. A dielectric substrate has a surface on which radio frequency circuits and microstrip lines are formed. At least one MMIC chip opening is dimensioned for receiving therethrough a MMIC chip. A metallic carrier is mismatched as to coefficient of thermal expansion to the dielectric substrate and includes a component surface adhesively secured to the dielectric substrate on the surface opposing the radio frequency circuits and microstrip lines. At least one raised pedestal is on the component surface that is positioned at the MMIC chip opening. A MMIC chip is secured on the pedestal and extends through the MMIC chip opening for connection to the radio frequency circuits and microstrip lines. Stress relief portions are formed in the metallic carrier that segment the carrier into subcarriers and provide stress relief during expansion and contraction created by temperature changes.

RELATED APPLICATION

[0001] This application is a continuation-in-part of patent applicationSer. No. 09/933,128 filed Aug. 20, 2001, the disclosure which is herebyincorporated by reference.

FIELD OF THE INVENTION

[0002] This invention relates to microwave monolithic integratedcircuits (MMIC), and more particularly, this invention relates tointerfaces and carriers used for mounting MMIC chips such as used inmillimeter wave modules (MMW).

BACKGROUND OF THE INVENTION

[0003] Millimeter wave (MMW) modules are becoming more commonplace asincreasing use is made of millimeter wave transceivers and similarmillimeter wave devices. Often, these modules are used with varioustransceiver designs having different transmitter and receive circuitsthat make use of a number of different microwave monolithic integratedcircuit (MMIC) chips or die. Many of the MMIC chips are formed fromGalium Arsenide (GaAs). These chips are often attached on an alumina,i.e., aluminum oxide, or similar dielectric substrate. Because of theextreme tolerances and necessity for thermal matching, millimeter wavemodules typically use an expensive coefficient of thermal expansion(CTE) matched housing material, such as copper tungsten (CuW) oraluminum silicon carbide (AlSiC) to mount Galium Arsenide MMIC chips andthe alumina or similar dielectric substrates. CTE matching is requiredto prevent the MMIC chips and the substrates on which they are attachedfrom cracking as the housing material shrinks and expands during extremetemperature variations.

[0004] Most millimeter wave Galium Arsenide MMIC dies or chips and theaccompanying ceramic substrates have a Coefficient of Thermal Expansion(CTE) that is between about 4 and 7 ppm/deg. centigrade (in someinstances, closer to between about 4 and 6 ppm/deg. centigrade). Thishas required the use of similarly matched housing materials, such as thecopper tungsten or aluminum silicon carbide materials as a metal carrier(or base plate of a housing) for heat sinking and attachment.Unfortunately, the lower cost housings, such as formed from aluminum,copper and other similar metallic or other materials, have a very highcoefficient of thermal expansion, greater than about 16 ppm/deg.Centigrade, and therefore, cannot be used. The use of CTE matchedcarrier material has been required in the past to prevent the chips andsubstrates from cracking as the carrier material shrinks and expandsversus temperature.

[0005] In some prior art modules, an epoxy preform is used to attach aceramic substrate having MMIC chips to a carrier. Compliant epoxy hasbeen used extensively in the past to attach CTE mismatched materials.The compliant characteristics of the epoxy allows it to have someelasticity, which enables the mismatched materials to expand atdifferent rates without being separated. However, every compliant epoxyhas a limited amount of elasticity, which limits the size of the bondedmaterial to a few mils. The size of the bonding area has been restrictedby the amount of CTE and mismatch. For example, the higher the CTEmismatch, the smaller is an allowable compliant epoxy bonding area.

[0006] An example of using non-CTE matched material that is less in costis disclosed in the incorporated by reference Ser. No. 09/933,128 patentapplication and provides a unique structure and method to interface ahousing and substrate material, both having a different coefficient ofthermal expansion, without damaging or impacting the performance of theMMIC chip, RF interconnects and other material components. Themillimeter wave (MMW) module for the microwave monolithic integratedcircuit (MMIC) includes a carrier board formed of a dielectric materialand having at least one MMIC die (chip) mounted thereon, and at leastone interface line. A base plate is formed of a material that has ahigher, unmatched coefficient thermal expansion (CTE) than the carrierboard. The base plate supports the carrier board.

[0007] A housing is mounted over the carrier board and engages the baseplate. This housing has at least one waveguide or subminiature coaxialconnector (SMA) interface mounted thereon. A flexible circuitinterconnect connects the subminiature coaxial connector(s) and the MMICdie through the interface line. A thermal interface member is positionedbetween the carrier board and base plate to aid in heat transfer betweenthe base plate and housing and the lower CTE carrier board.

[0008] The flexible circuit interconnect could be formed as one of fuzzbuttons or spring loaded self-adjusting interconnects, including the useof modified forms of pogo pins and similar spring segments and resilientmembers. The carrier board preferably, but not necessarily, comprises aplurality of layers of low temperature transfer tape (LTTT) to form amultilayer substrate board. The base plate and housing are formed from amaterial such as aluminum and/or similar metallic material. The thermalinterface member comprises a heat transfer gasket that is formed fromone of at least a phase change material, thermally conductive elastomer,or thermally conductive paste. Fasteners can secure the base plate andhousing together.

[0009] Although this type of assembly solves the interface problem toallow the floating of a ceramic or other board material relative to ahousing, including a base plate or carrier, there are still improvementsthat can be made by applying a MMIC assembly onto a CTE matched carrier.Some prior art techniques use an alumina substrate where Galium arsenideor other MMIC chips are attached directly to the CTE matched materialeither with solder or with silver epoxy. To provide efficient coolingfor the MMIC chips, holes are cut through the ceramic and raisedpedestals on the CTE matched carrier are used to directly attach theMMIC chips. The pedestals are also used to maintain the mounting surfaceof the MMIC chips at approximately the same level as the top portion ofthe ceramic board where all the radio frequency (RF) circuits andmicrostrip lines are present. In many chip and wire applications,co-planarity of the chip surface and the microstrip line is critical forradio frequency performance. Although the pedestals work well, themachining used to raise these areas is expensive because tighttolerances are required for precisely inserting the pedestals into cutholes on the ceramic board and provide a flat surface for mounting thechips.

[0010] It would be advantageous to replace a CTE matched carrier with anon-CTE matched carrier that is less expensive without having crackingand carrier material shrinkage or expansion versus temperature whilealso allowing the use of a large carrier.

SUMMARY OF THE INVENTION

[0011] The present invention advantageously sections a carrier intosmaller subcarriers and eliminates the size limitation. In effect, thecarrier has no size limitation. The present invention provides amicrowave monolithic integrated circuit (MMIC) assembly where adielectric substrate such as formed from alumina or other ceramicmaterial has a surface on which radio frequency circuits and microstriplines are formed. At least one MMIC chip opening is dimensioned forreceiving therethrough a MMIC chip. A metallic carrier having amismatched coefficient of thermal expansion to the dielectric substrateincludes a component side or surface, which is secured to the dielectricsubstrate on the side (surface) opposing the radio frequency circuitsand microstrip lines. The metallic carrier has at least one raisedpedestal on the component side that is positioned at the MMIC chipopening. A MMIC chip secured on the pedestal and extends through theMMIC chip opening for connection to the radio frequency circuits andmicrostrip lines. Stress relief portions are formed in the metalliccarrier that segment the carrier into subcarriers and provides stressrelief during expansion and contraction created by temperature changes.

[0012] In one aspect of the present invention, the MMIC chip includes acircuit connection surface wherein the pedestal is dimensioned such thatthe circuit connection surface of the MMIC chip is positioned coplanarwith the radio frequency circuits and microstrip lines on the dielectricsubstrate. The stress relief portions can be formed as grooves withinthe side of the metallic carrier opposite the component side or formedas cuts that extend through the carrier.

[0013] In yet another aspect of the present invention, the metalliccarrier is formed substantially from copper or aluminum. The metalliccarrier has a coefficient of thermal expansion between about 16 andabout 17 ppm/degree centigrade and the MMIC chip and dielectricsubstrate has a coefficient of thermal expansion between about 6 andabout 7 ppm/degree centigrade. An adhesive is positioned on those areascorresponding to subcarriers for adhesively securing the substrate tothe carrier. In one aspect of the invention, the adhesive comprises acompliant epoxy.

[0014] In yet another aspect of the present invention, the stress reliefportions comprise etched portions in which the metallic carrier has beenremoved. The subcarriers can be formed by etching the metallic carrier.

[0015] A method aspect of the invention allows the interfacing of aceramic substrate, at least one microwave monolithic integrated circuit(MMIC) and metallic carrier having a coefficient of thermal expansion(CTE) that is not matched with a ceramic substrate and a MMIC. Themethod comprises the steps of segmenting the carrier with stress reliefportions to form subcarriers and bonding the carrier with the ceramicsubstrate by an adhesive positioned at the subcarriers such that thestress relief portions and formed subcarriers provide stress reliefduring expansion and contraction created by temperature changes.

[0016] In yet another aspect of the present invention, the step ofsegmenting the carrier comprises the step of etching the carrier to formthe stress relief portions. The segmenting can be accomplished byforming grooves on the carrier or by forming cut lines that extendthrough the carrier for segmenting the carrier into subcarriers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Other objects, features and advantages of the present inventionwill become apparent from the detailed description of the inventionwhich follows, when considered in light of the accompanying drawings inwhich:

[0018]FIG. 1 is an exploded isometric view of a prior art millimeterwave (MMW) module having a CTE matched housing.

[0019]FIG. 1A is an exploded, isometric view of a prior art MMIC chipassembly with a CTE matched carrier where the alumina substrate andGalium Arsenide chips are attached directly to the CTE matched materialusing raised pedestals and holes.

[0020]FIG. 2 is an exploded isometric view of a housing with waveguideand SMA interfaces and corresponding base plate, and a dielectricsubstrate material as a carrier board for MMIC chips, the housing andcarrier base plate having different coefficients of thermal expansionthan the ceramic substrate and MMIC chip.

[0021]FIG. 3 is an enlarged isometric view of a board formed of adielectric material such as ceramic of the type that could be used withthe present invention.

[0022]FIGS. 4A and 4B are isometric views of the respective componentand backside of a large carrier that is segmented into smaller sectionsusing stress relief grooves on the backside and showing raised pedestalsthat are etched on the component side.

[0023]FIGS. 5A and 5B are isometric views of a large carrier segmentedinto smaller sections and showing stress relief cuts that extend fromthe backside to the component side with subcarrier sections.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] The present invention will now be described more fullyhereinafter with reference to the accompanying drawings, in whichpreferred embodiments of the invention are shown. This invention may,however, be embodied in many different forms and should not be construedas limited to the embodiments set forth herein. Rather, theseembodiments are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the invention to thoseskilled in the art. Like numbers refer to like elements throughout.

[0025]FIG. 1 illustrates a prior art millimeter wave (MMW) module 10,such as used for a millimeter wave RF transceiver or similar device,with a housing 12 formed of a CTE matched material, including a metalcarrier surface 12 a and protective housing 12 b. This housing is formedfrom a CTE matched material, such as copper tungsten (CuW) or aluminumsilicon carbide (AlSiC), and used to mount Galium Arsenide microwavemonolithic integrated circuit (MMIC) die or chips 14 with associatedalumina or similar ceramic or other material substrates used as acarrier board 16. RF signal lines 17 connect various elements as knownto those skilled in the art. The housing 12 is expensive and includesthe various coaxial or other cable connectors 18 and machined chip andsubstrate receiving channels 20, as illustrated. The coefficient ofthermal expansion for the housing and substrate/MMIC chip is closelymatched. In this type of design, the CTE match prevents the MMIC chipsand die and associated carrier boards, including single and multilayersubstrates, from cracking as the housing material shrinks and expandsduring temperature fluctuations.

[0026]FIG. 1A illustrates a prior art MMIC assembly with a CTE matchedcarrier. The alumina or other ceramic substrate 21 is a circuit boardand Galium Arsenide or other MMIC chips 22 are attached directly to theCTE matched carrier material 23 either with solder or with silver epoxy.To provide efficient cooling for the MMIC chips 22, holes 24 are cutthrough the alumina or other ceramic substrate (board) 21 and raisedpedestals 25 on the CTE matched carriers are used to directly attach theMMIC chips 22. Alignment holes 27 a aid in aligning the substrate to thecarrier using alignment mechanisms known to those skilled in the art.Mounting holes 27 b aid in mounting the carrier to a chassis or othermeans by attachment mechanisms known to those skilled in the art. Thepedestals 25 are also used to maintain the mounting surface of the chipsat approximately the same level as the top of the alumina or ceramicboard where all are radio frequency (RF) circuits and microstrip lines26 are present. In a typical chip and wire application, co-planarity ofthe chip surface and a microstrip line is critical for radio frequencyperformance. These pedestals are typically machined, but are expensivebecause of the type of tolerances required for precisely inserting thepedestals into the cut holes on the ceramic or aluminum board and alsoto provide the necessary flat surface for mounting MMIC chips. As isconventional, the carrier 23 can be formed from aluminum silicon carbide(AlSic) or copper tungsten (CuW) for CTE matching.

[0027] It is well known that any type of compliant epoxy or othermaterial used in this type of prior art application has a restriction onthe size of any carrier or two surfaces that can be applied to eachother, especially when there is a CTE mismatch. For example, in atypical application specific chart for compliant epoxy types such asfrom DIEMAT, the disclosure which is hereby incorporated by reference,it is evident that different epoxy types such as a DIEMAT 4030LD couldbe used to epoxy a ceramic board to a copper carrier (delta TCE=10). Thelargest dimension, however, of the two materials can be as high as 1.4inch in diagonal or a 1×1 inch square. The chart sets forth that largersurfaces cannot be supported. It is well known that compliant epoxy hasbeen extensively used in the past to attach CTE mismatched materials.Those skilled in the art recognize that the compliant characteristics ofthis epoxy allows it to have some elasticity to enable mismatchedmaterials to expand at different rates without being separated. Everycompliant epoxy, however, has a limited amount of elasticity whichlimits the size of the bonded material to a few mils as shown in theincorporated by reference DIEMAT chart. The present invention segments alarge carrier into smaller subcarriers and eliminates this sizelimitation. Thus, in effect, the carrier has no size limitation.

[0028] The present invention is advantageous and uses metal thinning andsegmentation techniques to allow the bonding of large surface areas suchas with compliant epoxy with large CTE mismatch. The segmentation andthinning of a carrier base plate (or carrier) provides stress reliefduring expansion and contraction due to temperature changes. Forexample, as shown in FIGS. 4A, 4B, 5A and 5B, and explained in detaillater, a large carrier is segmented into smaller sections. The costlymachining of the pedestals is replaced by low cost chemical etching ofcopper plate, such as a nickel plated copper plate, as a carrier baseplate and having a CTE between about 16 and 17 ppm/degrees C. Inaddition to the etching of the pedestal, the backside of the carrier isetched and divided into smaller subcarriers, also thinning out thicknessof the material. The alumina or ceramic board is attached to thesubcarrier by using compliant epoxy, such as DIEMAT 4030LD. The MMICchips are attached directly to the copper carrier on the top of thepedestals, using a compliant epoxy with adequate thermal properties,such as Diemat 6030HK. The compliant epoxy provides improved thermaltransfer without the need for soldering.

[0029] For purposes of background, a millimeter wave module (MMW) for amicrowave monolithic integrated circuit (MMIC) with an improvedinterface for a carrier base plate and housing cover as set forth in theincorporated by reference Ser. No. 09/933,128 application is firstexplained and described with reference to FIGS. 2 and 3 and followed bya description of the present invention.

[0030]FIG. 2 is an exploded isometric view of a Millimeter Wave (MMW)module 30 of the present invention, and showing a housing 32 and carrierbase plate 34, and dielectric substrate forming a circuit board 36 (orcarrier board by some skilled in the art). The housing and carrier baseplate have different coefficient of thermal expansions than the circuitboard. An interface is provided between the high CTE material, such asaluminum for the housing 32 and carrier base plate 34, and the low CTEmaterial, such as an alumina circuit board 36, without impactingperformance.

[0031] As illustrated, the circuit board 36 is formed of a dielectricmaterial, such as alumina or other ceramic material, and has at leastone MMIC die (chip) 38 mounted thereon and other surface mounttechnology (SMT) components 39. At least one interface line 40 isconnected to the MMIC chip and other surface mount technologycomponents. The interface line 40 is formed typically as a 50 Ohmmicrostrip interface line on the circuit board by fabrication techniquesknown to those skilled in the art. This circuit board 36 “floats”relative to the CTE mismatched carrier base plate 34 and housing 32,formed of a material having a higher and unmatched coefficient ofthermal expansion (CTE) than the ceramic board and supporting same.Other cable or interconnect components 41 can be mounted on circuitboard 36 as illustrated.

[0032] In one aspect of the present invention, the carrier base plate 34is formed from a low cost alumina, a similar aluminum alloy, or othersimilar commercially available material that is low cost and desirablefor these module applications. As illustrated, the MMIC die or chips 38are preferably attached directly to the top of the circuit board 36,which is formed as an alumina board or similar material and having theCTE mismatch from the aluminum carrier base plate, but similar to theGaAs MMIC devices. The housing 32 is formed from aluminum or similarmaterial and cooperates with the carrier base plate and mounted over thecircuit board 36 and engages the carrier base plate 34. The housing 32has at least one waveguide or subminiature coaxial connector interfacemounted thereon, such as a waveguide and subminiature coaxial connector(SMA) 42 interface, as known to those skilled in the art.

[0033] Subminiature coaxial connectors (SMA) are commonly used by thoseskilled in the art and are known as semi-precision, subminiature devicesthat are used with coaxial cables, including flexible and semi-rigidcabling. They are used up to about 18 GHz with semi-rigid cabling, andwith flexible cable, the SMA connectors are typically from DC values toabout 12.4 GHz. The SMA connectors are operable at broadband frequenciesand have low reflections. They can be designed to have a constant 50 Ohmimpedance, as known to those skilled in the art. Many different type ofSMA connectors are commercially available through many differentcompanies, including Light House Technologies, Inc. of San Diego,Calif., and Johnson Components of Waseca, Minn. They are available inpressure crimp, clamp and solder terminal attachments and provideinterconnections from various board striplines to coaxial cable, asknown to those skilled in the art.

[0034] The housing 32 is typically formed of a material having acoefficient of thermal expansion (CTE) similar to the carrier base plate34, and as noted before, can be formed of the lower cost aluminum. Aflexible circuit interconnect 44 (or interface) connects the waveguideor subminiature coaxial connector 42 and the MMIC die 38 through themicrostrip (preferably 50 Ohm) interface line 40 and allows the circuitboard 36 to shift relative to the housing 32 without signal degradation.Thus, a flexible interface is provided between the RF signal on thecarrier board and any coaxial connectors, such as the SMA connectors,positioned on the housing.

[0035] Although fuzz buttons 44 are used in one aspect of the presentinvention for the flexible circuit interconnects, other spring-loadedself-adjusting interconnects can be used. Many different types of fuzzbuttons are commercially available and can be modified for use with thepresent invention. One type of fuzz button is a gold plated molybdenumwool that fills passages through a material to provide conductivepathways. One size example is about 0.010 inch diameter by 0.060 inch inlength. Examples of fuzz buttons are disclosed in U.S. Pat. Nos.5,552,752; 5,631,446; 5,146,453; 5,619,399; 5,834,335; 5,886,590;6,192,576; and 5,982,186. These and any other fuzz buttons can bemodified to be operable with the present invention. Other types ofspring-loaded self-adjusting interconnects can also be used, includingmodified pogo pin connectors, which can include wires, pins or cablesformed as spring segments or other resilient members. Examples ofvarious types of pogo pins are disclosed in U.S. Pat. Nos. 6,252,415;6,242,933; 6,137,296; 6,114,869; 6,079,999; 5,451,883; and 5,948,960.

[0036] At least one alignment member 50 is mounted on the carrier baseplate 34. The circuit board 36 has a guide receiver 52 that receives thealignment member 50 for aligning the circuit board relative to thecarrier base plate without damage due to CTE mismatch. In one aspect ofthe present invention, the alignment member comprises an alignment pinthat aligns with a guide hole or notch.

[0037] As illustrated, a thermal interface member 60 is positionedbetween the circuit board 36 and carrier base plate 34 to aid in heattransfer between the base plate and housing and the lower CTE carrierboard. This thermal interface member can be formed as a heat transfergasket and can be formed from one of at least a phase change material,thermally conductive elastomer, or thermally conductive paste. This aidsin supporting the heat transfer and the “floating” of the board.Although not illustrated in detail, an EMI gasket can be positionedbetween the circuit board and housing or top cover. It does not providea mechanically adhesive interface. The two components float relative toeach other, allowing even greater CTE mismatches than possible withepoxy. It also facilitates rework.

[0038] Fasteners 62 are used to hold the carrier base plate 34 andhousing 32 mounted over the circuit board 34 together as a unit. Thefasteners 62 can be any type of fastener suggested by those skilled inthe art, but in the illustrated embodiment, are shown as screws. Thisparticular structure is well designed for a millimeter wave transceiverdesign, such as disclosed in commonly assigned U.S. patent applicationSer. No. 09/862,982, filed May 22, 2001. This type of transceiver designcan include a number of different MMIC chips and a carrier board that isformed as a plurality of layers of low temperature transfer tape (LTTT)that form a multilayer substrate board using low temperature co-firedceramic material, modified and formed as low temperature transfer tapeusing fabrication, printing and manufacturing techniques known to thoseskilled in the art.

[0039]FIG. 3 illustrates one type of board that can be used with thepresent invention. As illustrated, a substrate board 80 supportsdifferent layers of low temperature transfer tape (green sheets)technology sheets, including a DC signal layer 82, a ground layer 84,embedded capacitors and resistors layer 86, solder preform layer 88, andtop layer 90. The substrate board 80 provides support for the variouslayers. Cut-outs 92 can be formed with appropriate interconnects andconductive vias, signal lines and other components included within theoverall board 80 structure.

[0040] In accordance with the present invention, FIGS. 4A, 4B, 5A and 5Billustrate two embodiments of the present invention where a CTE matchedcarrier, as shown in FIG. 1A prior art, is now replaced with a largenon-CTE matched carrier 100, such as made from aluminum, copper,nickel-plated copper or similar material. A large, mismatched carriercan be used because of the advantageous segmentation into subcarriers.It is known that the coefficient of thermal expansion for copper andrelated materials is between about 16 and 17 ppm/degrees C, while GaliumArsenide (GaAs) MMIC chips and alumina or other ceramic substrates usedtherewith have coefficient of thermal expansions between about 6 and 7ppm/degree C. Because of the CTE mismatch, more expensive coppertungsten (CuW) or aluminum silicon carbide (AiSiC)have been used as theCTE matched carrier for these alumina or other ceramic substrates andMMIC chips. These CTE matched materials have traditionally been attachedby solder or silver epoxy. It is known, however, that the contactsurface between the two different CTE matched materials is made small,compliant epoxy can be used to bond the two surfaces. FIGS. 4A, 4B, 5Aand 5B illustrate how a large carrier, such as shown in FIG. 1A, issegmented into smaller sections. The carier in one aspect of theinvention is formed substantially from copper or aluminum, and could bea nickel plated copper. The costly machining of pedestals has beenreplaced by the lower cost chemical etching of copper to form pedestals104 using techniques known to those skilled in the art. Through holes105 provide part alignment and other means as suggested to those skilledin the art. The etched component side 106 shown in FIG. 4A illustratesthe etched pedestals 104 in accordance with the present invention.

[0041] In addition to etching of the pedestals, the backside 108, asshown in FIG. 4B, can be etched and subdivided into smaller sizesubcarriers 110 while also thinning out thickness of the material.Typically, these carriers 100 are a few mils thick and the etching willbe controlled by techniques known to those skilled in the art. Thealumina or other ceramic circuit board 112 is adhesively attached tothose areas defined by the copper subcarriers, some having therespective pedestals on the component side 100 a, using a compliantepoxy 114 such as DIEMAT 4030LD. The MMIC chips 116 are attacheddirectly to the carrier 100 on top of the pedestals, using a compliantepoxy 115 with adequate thermal properties, such as DIEMAT 6030HK.Techniques known to those skilled in the art are used for applying thecompliant epoxy and the board and chips. The compliant epoxy providesimproved thermal transfer without a requirement for soldering.

[0042]FIGS. 4A and 4B illustrate how an etchant is used to form stressportions as grooves 120 of a mil or a few mils deep on the backside 100b to subdivide the carrier 100 into smaller sections and form thesubcarriers 110. As illustrated, the stress relief grooves are linearlines that intersect each other and form rectangular subcarriers.Naturally, other geometric configurations could be used.

[0043]FIGS. 5A and 5B illustrate that an etchant or other means can beused to etch all the way through the carrier and form stress relief cuts130 with connecting joints 132 at the intersections of linear cut lines.It is evident that the subcarriers 110 are “floating” relative to eachother by the formation of stress relief cuts. The stress relief providedby the grooves or cuts and “floating” subcarriers allows a total surfacearea that will be secured and subject to CTE mismatch to be muchequivalently smaller than the total carrier, and thus, allow epoxyattachment between mismatched materials, such as the copper carrier,alumina substrate, and MMIC chips. Other types of grooves or cuts assuggested by those skilled in the art can be used in the presentinvention.

[0044] Many modifications and other embodiments of the invention willcome to the mind of one skilled in the art having the benefit of theteachings presented in the foregoing descriptions and the associateddrawings. Therefore, it is to be understood that the invention is not tobe limited to the specific embodiments disclosed, and that themodifications and embodiments are intended to be included within thescope of the dependent claims.

That which is claimed is:
 1. A microwave monolithic integrated circuit(MMIC) assembly comprising: a dielectric substrate having a surface onwhich radio frequency circuits and microstrip lines are formed and atleast one MMIC chip opening dimensioned for receiving therethrough aMMIC chip; a metallic carrier having a mismatched coefficient of thermalexpansion to the dielectric substrate, and a component surface which issecured to the dielectric substrate on the surface opposing the radiofrequency circuits and microstrip lines, and having at least one raisedpedestal on the component surface that is positioned at the MMIC chipopening; a MMIC chip secured on the pedestal and extending through theMMIC chip opening for connection to the radio frequency circuits andmicrostrip lines; and stress relief portions formed in the metalliccarrier that segment the carrier into subcarriers and provides stressrelief during expansion and contraction created by temperature changes.2. A microwave monolithic integrated circuit (MMIC) assembly accordingto claim 1, wherein said MMIC chip includes a circuit connection surfaceand said pedestal is dimensioned such that the circuit connectionsurface of the MMIC chips is positioned coplanar with the radiofrequency circuits and microstrip lines on the dielectric substrate. 3.A microwave monolithic integrated circuit (MMIC) assembly according toclaim 1, wherein said stress relief portions are formed as grooveswithin the side of the carrier opposite the component side.
 4. Amicrowave monolithic integrated circuit (MMIC) assembly according toclaim 1, wherein said stress relief portions are formed as cuts thatextend through the carrier.
 5. A microwave monolithic integrated circuit(MMIC) assembly according to claim 1, wherein said carrier is formedsubstantially from copper or aluminum.
 6. A microwave monolithicintegrated circuit (MMIC) assembly according to claim 1, wherein saidcarrier has a coefficient of thermal expansion between about 16 andabout 17 ppm/deg Centigrade and said MMIC chip and dielectric substratehave a coefficient of thermal expansion of between about 6 about 7ppm/deg Centigrade.
 7. A microwave monolithic integrated circuit (MMIC)assembly according to claim 1, and further comprising an adhesivepositioned on an area defined by the subcarriers for adhesively securingthe substrate to the carrier.
 8. A microwave monolithic integratedcircuit (MMIC) assembly according to claim 7, wherein said adhesivecomprises a compliant epoxy.
 9. A microwave monolithic integratedcircuit (MMIC) assembly according to claim 1, wherein said stress reliefportions comprise etched portions in which the metallic carrier has beenremoved.
 10. A microwave monolithic integrated circuit (MMIC) assemblyaccording to claim 9, wherein said subcarriers are formed by etching themetallic carrier.
 11. A microwave monolithic integrated circuit (MMIC)assembly comprising: a dielectric substrate having a surface on whichradio frequency circuits and microstrip lines are formed and at leastone MMIC chip opening dimensioned for receiving therethrough a MMICchip; a metallic carrier having a mismatched coefficient of thermalexpansion to the dielectric substrate, a component surface and acompliant epoxy thereon for adhesively securing the dielectric substrateon the surface opposing the radio frequency circuits and microstriplines, and having a plurality of raised pedestals on the componentsurface that are each positioned at a respective MMIC chip opening; aMMIC chip secured on a pedestal and extending through a respective MMICchip opening for connection to the radio frequency circuits andmicrostrip lines; and stress relief lines etched in the metallic carrierthat segment the carrier into rectangular configured subcarriers onwhich the pedestals are formed and provide stress relief duringexpansion and contraction created by temperature changes, wherein thecompliant epoxy is positioned at an area on the carrier defined by thesubcarriers.
 12. A microwave monolithic integrated circuit (MMIC)assembly according to claim 11, wherein said MMIC chip includes acircuit connection surface and said pedestal is dimensioned such thatthe circuit connection surface is positioned coplanar with the radiofrequency circuits and microstrip lines on the dielectric substrate. 13.A microwave monolithic integrated circuit (MMIC) assembly according toclaim 11, wherein said stress relief lines are formed as etched grooveswithin a surface of the metallic carrier opposite the component surface.14. A microwave monolithic integrated circuit (MMIC) assembly accordingto claim 11, wherein said stress relief lines are formed as cuts thatextend through the carrier.
 15. A microwave monolithic integratedcircuit (MMIC) assembly according to claim 11, wherein said carrier isformed substantially from copper or aluminum.
 16. A microwave monolithicintegrated circuit (MMIC) assembly according to claim 11, wherein saidcarrier has a coefficient of thermal expansion between about 16 andabout 17 ppm/deg Centigrade and said MMIC chip and dielectric substratehave a coefficient of thermal expansion of between about 6 about 7ppm/deg Centigrade.
 17. A method of interfacing a ceramic substrate, atleast one microwave monolithic integrated circuit (MMIC) and metalliccarrier having a coefficient of thermal expansion (CTE) that is notmatched with the ceramic substrate and the MMIC comprising the steps of:segmenting the carrier with stress relief portions to form subcarriers;and bonding the carrier with the ceramic substrate by an adhesivepositioned at an area defined by the subcarriers such that the stressrelief portions and formed subcarriers provide stress relief duringexpansion and contraction created by temperature changes.
 18. A methodaccording to claim 17, wherein the step of segmenting the carriercomprises the step of etching the carrier to form the stress reliefportions.
 19. A method according to claim 17, and further comprising thestep of forming grooves on the carrier for segmenting the carrier intosubcarriers.
 20. A method according to claim 17, and further comprisingthe step of forming cut lines through the carrier for segmenting thecarrier into subcarriers.